CPU processing power has continued to double on the average of every two years over the past twenty years. However, the performance of mass storage devices and computer bus interconnections between mass storage devices and CPUs has not increased at the same rate. As a result, performance in modem computer systems may be limited by the speed at which data can be accessed and transported between memory and peripheral devices, such as hard disk drives. In addition, computer systems may be limited by the amount of data that they can efficiently access.
The small computer system interface (“SCSI”) bus is the most widespread and popular computer bus for interconnecting mass storage devices, such as hard disks and CD-ROM drives, with the memory and processing components of computer systems. Depending on the particular SCSI standard used, the SCSI bus may interconnect 7, 15, or 31 different mass storage devices with an internal bus within a computer system. Data transfer rates may range from 2 megabytes (“Mbytes”) per second up to 40 Mbytes per second.
Significant limitations result in SCSI systems from the relatively small number of available bus connections, and from the fact that the bus connections are typically only accessible from a single computer system. These factors limit the amount of data available to a computer system, and the rate at which data can be transferred between a mass storage device and a computer system.
The fibre channel architecture and protocol for data communications has been developed in order to overcome the limitations imposed by the SCSI bus architecture. When optical fibers are employed as a physical medium for interconnecting computer systems and mass storage devices, a fibre channel network can extend for ten kilometers and can transfer data between a computer system and a mass storage device at up to 200 Mbytes per second. Fibre channel technology also provides network topologies and an addressing scheme which permit configurations that are much more powerful and flexible than those available using SCSI technology. Whereas the SCSI bus supports connection of up to 31 target devices, a fibre channel network can support connection of more than 16,000,000 target devices.
The fibre channel arbitrated loop (FC-AL) topology is the most commonly employed fibre channel topology. Using existing encoding and addressing schemes, a single FC-AL supports interconnection of up to 127 fibre channel devices. Fewer devices are used in practice, however, because propagation delay and general loop overhead increase as more and more devices are added to the loop. At some point, it is simply uneconomical to continue adding devices. Additional arbitrated loops may be used to support further devices, however this requires a fabric-based fibre channel topology, which is more complicated and expensive to implement than an individual arbitrated loop.